538 Scientific Proceedings, Royal Dublin Society. 
from ordinary solutions; they are merely saturated or non-saturated 
solutions of the anhydrous salt, combination to form the hydrate 
taking place only at the moment of crystallization. It will then 
be seen that the view put forward by Nicol is essentially different 
from mine in the following particulars. For reasons already 
stated it appears to me—lIst, that the anhydrous salt must become 
hydrated before it can enter into solution; 2ndly, that the hydrated 
compound, which is a crystalline salt existing in the so-called 
supersaturated solution, is not the hydrate that crystallizes out 
when a crystal is introduced; and 3rdly, that the phenomenon of 
supersaturation is a manifestation of differences in solubility of 
two distinct crystalline hydrates at a given temperature, which 
may be quite different in the extent of their hydration, and one 
of which may be formed from the other at the moment of erystal- 
lization by some considerable proportion, if not the whole, of the 
solvent water, entering into combination with the hydrate in 
solution, whereby the newly formed compound becomes insoluble 
and separates as a crystalline mass. 
My attention has been drawn to a more recent communication 
by Dr. Nicol, in which he returns to the subject of supersatura- 
tion of solutions of salts which do not combine with water to form 
crystalline hydrates. 
The question he states is generally regarded as undecided, not- 
withstanding that there are many excellent examples of super- 
saturation among organic compounds, from which the possibility 
of forming hydrates or analogous compounds is excluded, and con- 
sequently the explanation, which proves satisfactory for salts con- 
taining water, isin this case not applicable. It may be remarked 
here that chemists engaged in the manufacture of organic com- 
pounds have long since recognised the fact that these substances 
frequently exhibit the phenomenon of supersaturation, though in 
some cases at least it has been doubted whether they were not 
cases of superfusion. Phenol is a conspicuous example, as it is 
usual to crystallize it in large masses by the introduction of a 
ready formed crystal of the substance. 
There are other cases, such as that of potassium hydrogen 
tartrate, in which the substance is formed in solution, and 
1 Zeitschrift fir Anorganische Chemie, vol. xv., 1897, p. 397. 
